Electroless Nickel Plating for Corrosion Resistance
There’s nothing like a sudden failure to make one realise that having chosen the right coating for one’s part in the first place will save a world of subsequent costly repair. Here’s what needs to be known when choosing the electroless nickel coating for particularly corrosion sensitive parts.
Why the Deposit Blocks Corrosive Attack
Electroless nickel coatings provide uniform coverage across all surfaces of a part, including recesses and holes. This results in a 100% solid coating, devoid of pinholes, that acts as a physical barrier to corrosion between a substrate and the surrounding environment. This is in contrast to electroplated coatings that can have areas of varying thickness, such as holes at the edges of recesses, which can be prone to local attack.
Phosphorus Content and Protection Level
A deposit with a low phosphorus content (1% – 4%) offers good wear resistance; however, the corrosion resistance is only moderate. A high-phosphorus Electroless Nickel Coating with 10% – 13% phosphorus offers an amorphous, almost pore-free structure. This coating offers significantly better corrosion resistance than a low-phosphorus coating. The corrosion resistance is equivalent to 500 hours or more of salt spray testing.
Environments Where It Performs Well
The coating is resistant to corrosion from salt spray, dilute acids and alkalis in a humid atmosphere, and is widely used in the oil and gas, food processing, and electronics industries for parts subject to chemical splash, condensation, or mild acidic cleaning. It is not resistant to corrosion from strong oxidising acids, such as concentrated nitric acid, and compatibility with process media needs to be established.
Heat Treatment After Plating
Post-plate heat treatment to relieve hydrogen embrittlement in steels plated by the aqueous solution immersion process can start to crystallize the amorphous structure with consequent deterioration in corrosion resistance, as an alternative to increased hardness. Discussion with the plater regarding the need for such post-plate treatment for corrosion resistance as opposed to increased hardness would be appropriate for components that are to operate in aggressive corrosive environments.
Reading the Specification for Corrosion Duty
ASTM B733 classifies service conditions for electroless nickel plating into several classes based on the level of phosphorus content and minimum salt spray hours. These are known as the SC (Service Condition) classes and include: SC1 for light service, SC2 for general service, SC3 for moderately severe service, SC4 for severe service. A high-phosphorus deposit (such as 10% P) specified at 25 micrometers thickness would provide 500 hours or more before first evidence of corrosion in salt spray testing, making it suitable for the most demanding of corrosion environments.
A useful reference for Electroless Nickel Coating is www.poeton.co.uk/surface-treatments/plating/electroless-nickel-plating/.
Matching the grade, thickness and post-treatment of the deposited coating to the conditions actually experienced by the coated part will result in a coating that performs as promised rather than falls short of that promise.
